1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for increasing low-frequency content of seismic energy generated by a marine seismic source array.
2. Discussion of the Background
Reflection seismology is a method of geophysical exploration to determine the properties of a portion of a subsurface layer in the earth, information that is especially helpful in the oil and gas industry. Marine reflection seismology is based on the use of a controlled source that sends energy waves into the earth. By measuring the time it takes for the reflections to come back to plural receivers, it is possible to estimate the depth and/or composition of the features causing such reflections. These features may be associated with subterranean hydrocarbon deposits.
For marine applications, a seismic survey system 100, as illustrated in FIG. 1, includes a vessel 102 that tows plural streamers 110 (only one is visible in the figure) and a seismic source 130. Streamer 110 is attached through a lead-in cable (or other cables) 112 to vessel 102, while source array 130 is attached through an umbilical 132 to the vessel. A head float 114, which floats at the water surface 104, is connected through a cable 116 to the head end 110A of streamer 110, while a tail buoy 118 is connected, through a similar cable 116, to the tail end 1108 of streamer 110. Head float 114 and tail buoy 118 are used, among other things, to maintain the streamer's depth. Seismic sensors 122 are distributed along the streamer and configured to record seismic data. Seismic sensors 122 may include a hydrophone, geophone, accelerometer or a combination thereof. Positioning devices 128 are attached along the streamer and controlled by a controller 126 for adjusting a position of the streamer according to a survey plan.
Source array 130 has plural source elements 136, which are typically air guns. The source elements are attached to a float 137 to travel at desired depths below the water surface 104. During operation, vessel 102 follows a predetermined path T while source elements (usually air guns) 136 emit seismic waves 140. These waves bounce off the ocean bottom 142 and other layer interfaces below the ocean bottom 142 and propagate as reflected/refracted waves 144, which are recorded by sensors 122. The positions of both source elements 136 and recording sensors 122 may be estimated based on GPS systems 124 and recorded together with the seismic data in a storage device 127 onboard the vessel. Controller 126 has access to the seismic data and may be used to achieve quality control or even fully process the data. Controller 126 may also be connected to the vessel's navigation system and other elements of the seismic survey system, e.g., positioning devices 128.
A source element may be impulsive (e.g., an air gun) or vibratory. A vibratory source element is described in U.S. patent application Ser. No. 13/415,216 (herein the '216 application), filed on Mar. 8, 2012, and entitled, “Source for Marine Seismic Acquisition and Method,” assigned to the same assignee as the present application, the entire content of which is incorporated herein by reference.
Presently, the air gun is the work horse of marine seismic acquisition. However, neither air guns nor existing vibratory source elements are effective in the low-frequency range of the spectrum, mainly in the 1 to 10 Hz range. In other words, energy generated by an air gun in the low-frequency spectrum is too weak to be reflected to seismic receivers to be recorded. The low-frequency energy range is useful in seismic exploration because it provides better seismic energy depth penetration, which is extremely valuable for imaging complex geological settings, such as sub-salt, basalt, chalk or even dense carbonate. New processing methods, such as full wave inversion (e.g., a data-fitting procedure based on full-wavefield modeling to extract quantitative information from seismic data) can benefit greatly from increased energy in the low-frequency range.
Thus, there is a need to obtain the low-frequency range to better interpret seismic data. To be able to record such data, source arrays need to be adjusted/modified to generate low-frequency content. Therefore, it is desirable to provide source elements and methods capable of generating low-frequency energy.